Hello @thejbshop ,
It sounds like once you've paid the $79 to Costco, you then want to indicate that you actually have a credit with iTunes. If my assumption is correct, then you could do the following.
1. Post the purchase of $79 to Costco as you have done, but do not use an expense account. Use an Other Current Asset account. If you don't have one set up for this purpose, you could set it up and call it something like iTunes Credits. Post +$100.00 to the iTunes Credits asset account.
2. Post a 2nd line to either the expense account that you normally expense for iTunes purchases, or a separate Purchases Discounts account - your choice. Post -$21.00 to this account.
Now you have recorded your $79 purchase from Costco.
As you use the $100 in iTunes credits, for each transaction you will DR Expense and CR Asset, thus reducing the asset as you use it.
Any gift card or item such as this where you have paid in advance prior to using the product/service, should be handled similarly to what I've described above.
If you would rather eventually enter your purchases into a vendor bill using the iTunes vendor name, then you can accomplish the same thing as above by posting to Accounts Payable on the first line of your Costco transaction, and tabbing over to the name field and entering the iTunes vendor name. Each time you use a credit, you would post a bill to your iTunes vendor name and expense each transaction to your expense account.
Both ways will work, it just depends upon how you want to see things on your books. The 1st method will show that you have an asset on the books that you are reducing as you use your credits; the 2nd method will show that you have a negative liability (same as a positive asset) for iTunes vendor, which will increase as you use your credits.
